The operation targets insurgents from the Nusra Front group in the Juroud Arsal area on the outskirts of the Lebanese town of Arsal and areas near the Syrian town of Fleita, a Syrian commander told Reuters.

The area is home to about 1,000 militants, including Islamic State group militants, jihadists and other rebels fighting in Syria's six-year war. Several thousand Syrian refugees also live in the town of Arsal.

The offensive was widely expected after negotiations with militants failed over the past days.

On Tuesday, Lebanese Prime Minister Saad Hariri said the country’s military is preparing a carefully-planned military operation to secure a lawless section of the border with Syria, while Hezbollah leader Sheikh Hassan Nasrallah hinted in a speech last week that a joint operation was in the works with the Lebanese and Syrian militaries.

The battle was fought by Syrian troops and Hezbollah gunmen on the Syrian side of the border. At the same time, the Lebanese army fought against the militants on the Lebanese side.

The Syrian Central Military Media said Syrian troops and Hezbollah fighters captured some areas from the militants and killed and wounded a number of extremists.

There was no official indication on Friday on whether Lebanese troops were involved in the fighting. But the Lebanese National News Agency reported that the army had fired on a group of militants trying to flee the fighting toward Arsal town.

A Lebanese security source said refugees living in the area were fleeing toward Arsal and U.N. refugee agency, UNHCR, spokeswoman Lisa Abou Khaled said only a small number of people had fled to Arsal so far.

"UNHCR has only received confirmation ... that two Syrian families have arrived in the town of Arsal from the outskirts," she said.

Hezbollah said the border area has been used in the past to launch attacks deep inside Lebanon, including a wave of bombings since 2013 that have killed scores of people.

In 2014, more than two dozen Lebanese soldiers and policemen were seized by by militants from al-Qaeda and the Islamic State group who had crossed the border from Syria. Sixteen have since been released and four have been killed by their captors.